Omnidirectional loop antenna system



2 Sheets-Sheet 1 Filed Feb. 5, 1951 INVENTOR EMERICK TOTH TO ANTENNA TOANTENNA 2 ATTORNEYfi Sept. 28, 1954 E. TOTH 2,690,509

OMNIDIRECTIONAL LOOP ANTENNA SYSTEM Filed Feb. 5, 1951 2 Sheets-Sheet 2INVENTOR EM ER l CK TOTH ATTORNEYj Patented Sept. 28, 1954 i EHQEOMNIDIRECTIONAL LooP ANTENNA SYSTEM Emerick Toth, Takoma Park, Md.

Application February 5, 1951, Serial No. 209,502

2 Claims.

(Granted under Title 35, U. S. Code (1952),

sec. 266) This invention relates to omni-directional antenna systems.

More particularly this invention relates to omni-directional antennasystems made up of a plurality of directive antenna elements.

It is generally known that both the dipole and loop type antennas havedirectional characteristics. The three dimensional figure representingthe response pattern of these antennas is toroidal or doughnut-shaped.In the case of the loop antenna, the axis of the toroid is perpendicularto the plane of the loop, while with the dipole the said axis coincideswith the dipole.

It is also known in the art (see U. S. patent to Weagant 2,280,562) thatby taking two similar loop antennas or dipoles and relating the dipoleor loop antenna elements so that their directive patterns would be in a90 degree space phase relation and then displacing the phase of thevoltage induced in one of the antenna elements so that a spherical oromni-directional response pattern will result if the voltages thusobtained are added together for most conditions of wave polarization.

The problem of getting the 90 degree time phase displacement between thevoltages originating in the directive antenna elements was in the U. S.patent to Weagant 2,280,562 accomplished by loosely inductively couplingthe resonant circuit of which one directive element is a part to theresonant circuit of which the other directive element is a part, andthen taking the output across one of the purely reactive elementstherein.

The prior art methods have been found unsatisfactory when more than asingle frequency is to be simultaneously received, or when the receiveris not exactly tuned to the received frequency. Thus, for example, whenpulsed energy, or amplitude or frequency modulated waves are to bereceived wherein the received wave has a substantial bandwidth, theresponse pattern for all the frequencies other than those in thevicinity of the carrier frequency were substantially directive becausethe proper phase and amplitude relationships were obtained only at ornear the carrier frequency.

The present invention is an improvement over the prior art methods inthat omni-directional response is obtained over a substantially largerband of frequencies.

One aspect of the present invention basically consists of coupling theinductive-capacitive circuits associated with each of the directionalantennas so that the degree of coupling is in the neighborhood ofcritical coupling.

Another aspect of the present invention is in broadening the responsestill further by increasing the coupling slightly above the criticalvalue and then changing the sensitivity of one of the antenna elements,and then adding reactance as necessary to compensate for any change sothat the circuits are tuned substantially the same as before.

One object of the present invention is therefore to provide an antennasystem comprising a plurality of directive elements wherein the circuitsassociated therewith give the antenna system omni-directionalcharacteristics over a band of frequencies.

Still another object of the present invention is to provide anomni-directional antenna system over a band of frequencies made up ofdirective antenna elements by means of relatively simple circuitryassociated therewith.

These and other objects will become apparent to those skilled in the artupon reference to the specification to follow and the attached drawingswherein:

Figure 1a is a perspective view of the directive toroidal responsecharacteristic of the loop and dipole antenna.

Figure 1b is a right cross-sectional view of the toroidal characteristicof Figure 1a.

Figure 2 is one embodiment of the present invention.

Figures 3-5 show other embodiments of the present invention.

Figure 6 is a simplified embodiment of the present invention.

Figures 7a., b, and c disclose various response curves obtained when thecoupling is varied between the resonant circuits associated with eachantenna element of the embodiment of Figure 2.

Figure 7d is the response characteristic for the embodiment of Figure 5.

In the specification to follow, the same reference characters connotesimilar elements throughout.

As is well known in the art, a dipole or single wire antenna a (seeFigure 1a) has directive properties in that it is responsive to amaximum degree to an electric field parallel to it, and is notresponsive at all to an electric field perpendicular to it. In suchcase, the three-dimensional figure in Figure 1a represents the responsepattern of such an antenna.

Substantially the same toroidal response pattern is present for the loopantenna b where the axis of the pattern is perpendicular to the plane ofthe loop.

A right cross-section of the pattern 0 would be a figure 8 as shown inFigure 1b formed by two circular figures e and 1.

Two similar antennas whose outputs are added together and placed so thattheir responsive patterns are at right angles in space would at firstthrought appear to give an omni-directional system. It can be shown,however, that the system would only be non-directive when the phase ofthe voltage produced by one of the antennas is displaced 90 degrees withrespect to the other. To make the pattern omni-directional over a bandof frequencies, however, not only must the phase of one of thesevoltages remain displaced 90 degrees relative to the other, but therelative amplitudes of the voltages must remain the same regardless offrequency over at least a restricted band.

To obtain a relatively simple circuit which will shift the phase in theneighborhood of 90 degrees and at the same time maintain the samerelative amplitudes for a band of frequencies would appear to be no easymatter. However, the present invention accomplishes this result by theuse of relatively simple double tuned coupled circuits as shown inFigures 2, 3 and 4 wherein the degree of coupling used becomesimportant, as well as the relative gains of the antennas.

Basically one aspect of the present invention comprises twoinductive-capacitive circuits each including one of the directiveantennas which are coupled together so that one circuit is separatelytuned to a particular frequency which falls within the band offrequencies to be received, and the other circuit is separately tuned tothe same frequency.

The degree of coupling between the two circuits is at least equal to orgreater than critical coupling at the above mentioned frequency. Bycritical coupling is meant that degree of coupling between two circuitswhere the square of the coupling impedance is equal to the product ofthe equivalent series resistances for the two circuits.

At critical coupling the phase and amplitude voltage relationships areproper to give the antenna system substantially omni-directionalcharacteristics over a narrow band of frequencies for the usual valuesof circuit Q.

In the embodiment of Figure 2, two similar loop antennas are placed atright angles to each other and are respectively coupled to tuned circuitI and 2' through an impedance matching transformer 3 which serves alsoto coupled tuned circuit 2 to tuned circuit I. Tuned circuits I and 2'include respectively tuning condensers I and 8 coupled acrossinductances 56' which in the example shown are also part of thesecondary windings of transformer 3. The coupling between tuned circuitsI and 2' is adjusted to be at least equal to critical coupling. (Thedesirability and effect of varying the degree of coupling above criticalcoupling will be later discussed.) Condensers I and 8 are preferablyadjusted so that the circuits I and. 2 and the impedance respectivelyreflected therein from the primary circuits of transformer 3, whichincludes antennas I and 2, resonate to the same desired frequency.

The voltage across tuning condenser l in circuit I is coupled to acircuit to which the system is to be associated. Condenser I is showncoupled between the grid 9 and cathode It of an electron dischargedevice I? since the an tenna system there shown is used as a receivingantenna system. If antenna I-2 were to be than one.

used as a transmitting antenna system, circuit I would be coupled to theoutput of a transmitter device.

The embodiment of Figure 3 is similar to the embodiment of Figure 2except that the tuned circuits I and 2' are coupled directly togetheracross a mutual coupling impedance comprising a condenser I2, instead ofby a transformer 3 as in Figure 2. Accordingly, separate uncoupledtransformers 3 and 3 are utilized respectively to couple the antennas I2to tuned circuits I and 2. The circuit is preferably tuned so thatcondensers l, inductance 5", the reactance reflected from the primarycircuit of transformer 3' (which includes antenna I), and a capacityequal to /2 the value of condenser I2, form a circuit resonant to thedesired frequency. Likewise, condenser 8", inductance 6", the reactancereflected from the primary circuit of transformer 3 (which includes loopantenna 2), and a capacity the value of capacity I2 form a circuit alsoresonant to the desired frequency. The coupling between circuits I and 2is adjusted to at least critical coupling.

The embodiment of Figure 4 is similar to the embodiment of Figure 3except that a series condenser I2 is used to couple circuits I and 2'together. Condenser I2 is of such value that it offers a low impedanceto the operating frequency. Condensers I and 8 are preferably adjustedso that condensers i and 8 resonate respectively with inductance 58 andthe reactance reflected from the primary circuits of transformers i'4".

The embodiment of Figure 5 is similar to the embodiment of Figure 2except that the size of antenna 2 has been reduced so that the voltageat its terminals is less than that at antenna I. The decrease in thereactance of antenna 2 is compensated for by adding an inductance IIequal to the decrease of inductance of antenna 2. Instead of reducingthe size of the loop, the number of turns in the loop could be decreasedinstead if loop 2 comprised several turns rather The advantage of theembodiment 5 over the embodiment of Figure 2 is that it has substantialominidirectivity over a larger band of frequencies for reasons whichwill be later explained.

The circuit shown in Figure 6 is a simplified circuit which is similarto the embodiments of Figures 2 and 5 except that the impedance matchingtransformer coupling the antennas I and 2 to the tuned circuits I and 2'has been omitted. Accordingly circuit I' there shown includes a seriescircuit of loop antenna I, an inductance 5 which is inductively coupledto the inductance 6 of tuned circuit 2', and a tuning condenser I. Theresistance I3 shown in circuit I represents the total resistance of thecircuit and includes the resistance of the winding of inductance 5 etc.

Likewise, circuit 2' comprises a series circuit of an inductance 6 whichis coupled to inductance 5 of circuit I, antenna 2 and tuning condenser8. Resistance I4 shown in circuit 2 represents the total resistance incircuit 2'.

The voltage e7 across condenser I is coupled to load device I1. Theembodiment of Figure 6 because of its simplicity will be used to explainthe theory of operation of the present invention, it being understoodthat the embodiments of Figures 3-5 operate in a similar manner. Ifcircuit 2 were to be open circuited then circuit I Would be tuned to afrequency f0 preferably the carrier frequency. If the circuit I wereopen circuited then circuit 2 would be tuned to the same frequency fo.

Condenser voltage 21 is made up of two component voltages er and er. Inorder to render the antenna system non-directive for a given frequency,the phase of voltage 67 must lead or lag e1" by 90 degrees, and therelative amplitudes of er and er" must be the same as the relativeamplitudes of the voltages induced by the signal in two substantiallyidentical antennas (i. e., antennas having similar response) whosedirectivity patterns are 90 space degrees apart having sinusoidalresponse patterns as shown in Figure b.

The curves of Figur 7 illustrating the various response characteristicsof the double-tuned circuit of Figure 6 will now be referred to togetherwith Figure 6 to explain the theory of operation of the invention hereinclaimed used as a receiving antenna. It should be understood that by asimilar analysis, the operation of the antenna system when used as atransmitting antenna system will become apparent.

As was previously explained, the main object of the instant invention isto provide a substantially omnidirectional antenna system. If the netvoltage e7 across condenser I (which is coupled to load device I'I)remains constant irrespective of the direction from which the incomingradio waves strike antennas I and 2 then the antenna system isomnidirectional.

The voltage component e1" developed across condenser I of circuit I isproduced by the volttage e" present at the terminals of receivingantenna 2. The voltage component e1 developed across condenser I ofcircuit l' is produced by the voltage 6 present at the terminals ofreceiving antenna I.

Assume that the net voltage 6 and e" induced respectively in antennas Iand 2 are equal at frequency fo (the frequency to which circuit I istuned if circuit 2 were open circuited and vice versa). Now, antenna I,and thus voltage e is directly applied in circuit I. The voltage 6originating in antenna 2 is coupled to circuit I in Figure 6 by theinductive coupling between inductances 5 and 6. If the coupling is equalto critical coupling (for the circuit shown in Figures 2 and 6 this ispresent when the mutual impedance= /R1s R14) it has been found that thevoltage e7" across condenser I which originates from antenna 2 is 90degrees out of phase with the voltage 27' across condenser I whichoriginates from antenna I and the amplitudes of the voltages are equal.

Figure 7a shows the amplitude variation of the voltages er and er acrosscondenser I as the frequency is varied above or below frequency ft forthe condition of critical coupling. In this connection it should benoted that the e7 curve represents the primary circuit frequencyresponse curve of a double tuned circuit while the av curve representsthe secondary circuit frequency response curve of the same double tunedcircuit.

Between the center frequency f0 and frequencies f1 and f2 respectivelybelow and above center frequency, the amplitude differences between erand er" are not in general sufficiently different to cause much changein the omni-directive characteristics of the antenna system. Althoughthe phase difference between the voltages also varies from 90 degrees oneither side of center frequency f0, this change is not substantialenough within the range of frequencies from h to f2 to seriously varythe omni-directive characteristics of the antenna system.

If the coupling is reduced much below critical coupling two undesirableconditions result. First, as shown in Figure 7b, the amplitudedifference between er and er" becomes pronounced even at the centerfrequency in; also for a givenfrequency band extending between f1 and f2above and below the center frequency ,fo, the phase difference betweenew and er" varies more from the 90 degree phase condition than for thecase of critical coupling.

If the coupling is increased above critical coupling then the responsecurve of Figure results. Here the amplitude difference between thecurves of er and er" also becomes pronounced but it has been found thatthe greater the coupling the less becomes the phase difference betweenat and 61 from the desired degree phase relation condition for the sameband of frequencies.

If the portions of curve er of Figure 70 between the peak 0 and d werebrought down to the level of the portion between the peak of curve e7,or if the level of curve e7 was raised to meet the level of the portionof curve an" between the peaks as shown in Figure 7d, then betteromni-directivity would result over a given band of frequencies becausethe proper phase and amplitude conditions are present over a greaterfrequency range. Of course, if the circuits were overcoupled too muchabove critical coupling, the sensitivity or response would drop off somaterially as to be unsatisfactory from the sensitivity standpoint. Theseriousness of this loss of overall sensitivity depends on theparticular conditions under which the present invention is used.

A considerable advantage may be achieved from a small amount ofovercoupling from the critical coupling points without much loss ofoverall sensitivity.

The selectivity curves of 61' and c7" are raised or lowered byincreasing or decreasing voltage e or e". This is accomplished byincreasing or decreasing the overall gain or sensitivity of one of theantennas.

If a loopantenna is the directive element, this can be accomplished bydecreasing the size of loop 2 (see Figure 5) or increasing the size ofloop I. Varying the number of turns of the loop is another way to changethe overall gain or sensitivity of the loop.

In an embodiment such as shown in Figure 2 utilizing a transformer 3 tocouple the directive antennas I-2 respectively to the tuning circuits Iand 2', the selectivity curves e7 or e7 can be raised or lowered byvarying the step-up ratio (transfer efficiency) of the transformer.

Changing the gain or sensitivity of an antenna may result in a change ofits impedance so that reactance (inductance H in Figure 5) must be addedto compensate for the change of impedance so that circuits I and 2 arestill separately tuned to the same frequency.

It is to be noted that in the embodiment of Figure 5 just described,even though one of the antennas is not substantially identical to theother the condition for omni-directivity previously mentioned that therelative amplitudes of varying the values of the resistances in theserespective circuits. (I. e., vary resistances l3 and I4 shown in Figures2, 3, 4 and 6.) If the circuit is in a condition of critical coupling itcan be shown that a condition slightly above critical coupling may beobtained by slightly decreasing the value of either resistances 13 or M.

The embodiments of Figures 2-5 include transformer coupling between theloop antennas and the tuned circuits 1 and 2 to increase the systemsensitivity, and for impedance matching purposes. Varying the step upratio of transformer 3 also presents a convenient way to utilize thesame antennas for widely separated frequency bands since changing thestep up ratio of transformer 3 varies the impedance reflected intocircuits I and 2' and thus the tuning condenser therein can thereby tunethe associated circuits to resonance over several frequency hands.

If a completely omnidirectional (spherical) response pattern is notdesired, then the antennas may be placed at an angle other than 90degrees which will give the system an amount of directivity depending onthe degree to which the antennas are displaced from the 90 degreerelation.

Many modifications may be made of the specific embodiment disclosedwithout deviating from the scope of the broadest aspect of the presentinvention.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

1. In an omni-directional antenna system comprising first and seconddirective antennas, one antenna having a greater gain than the other andhaving the axes of their directivity patterns in degree space phaserelation, first and second parallel-resonant tuned circuits coupledrespectively to said first and second antennas and forming therewithrespective resonant circuits tuned to the same given frequency, mutualcoupling means coupling said parallel-resonant circuits together so thatthe degree of coupling therebetween is greater than critical coupling.

2. An omni-directional antenna system comprising first and seconddirective antennas, one antenna efiectively having a greater gain thanthe other and having the axes of their directivity patterns in 90 degreespace phase relation, first and second parallel-resonant tuned circuitscoupled respectively to said first and second antennas and formingtherewith respective resonant circuits tuned to the same givenfrequency, mutual coupling means coupling said parallel-resonantcircuits together so that the degree of coupling therebetween is greaterthan critical coupling, and means coupling only the voltage developedacross the parallel-resonant circuit associated with the antenna havingthe larger gain to a utilization circuit.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,280,562 Weagant Apr. 21, 1942 2,488,612 Tunick Nov. 22, 19492,520,984 Williams et a1 Sept. 5, 1950 FOREIGN PATENTS Number CountryDate 362,530 Great Britain Dec. 10, 1931 419,783 Great Britain Nov. 19,1934 532,164 Great Britain Jan. 20, 1941

